An electrodynamic transducer may be utilized as a loudspeaker or as a component in a loudspeaker system to transform electrical signals into acoustical signals. In a typical loudspeaker system, the transducer includes a magnetic motor assembly including one or more permanent magnets mounted between a top plate and a back plate, and a voice coil attached to a coil former and axially movable with respect to the motor assembly. The coil former and attached voice coil are inserted into an air gap of the motor assembly such that the voice coil is exposed to the magnetic field established by the motor assembly. The coil former is attached to a diaphragm constructed from a flexible material that is responsive to a vibrational input, such that the diaphragm is mechanically referenced to the voice coil.
During operation of the loudspeaker, electrical energy is supplied to the voice coil, causing the voice coil and attached diaphragm to move axially within the air gap. Electrical signals are transmitted as an alternating current through the voice coil, and the alternating current interacts with the constant magnetic field in the air gap. The interaction results in a Laplace force which is expressed as a product of the magnetic flux density, overall length of the turns of the voice coil linked to the magnetic flux, and the value of the electrical current running through the voice coil. Due to the Laplace force acting on the voice coil positioned in the magnetic field, the alternating current actuates the voice coil to reciprocate back and forth in the air gap and, correspondingly, move the diaphragm to which the coil former is attached. Accordingly, the reciprocating voice coil actuates the diaphragm to likewise reciprocate and, consequently, produce acoustic signals that propagate as sound waves.
Since the material of the voice coil has an electrical resistance, some of the electrical energy flowing through the voice coil is converted to heat energy instead of sound energy. Heat produced by the voice coil can build up and be radiated to surrounding surfaces of the transducer. The generation of resistive heat is disadvantageous for several reasons. First, the conversion of electrical energy to heat energy constitutes a loss in the efficiency of the transducer in performing its intended purpose, that of converting the electrical energy to mechanical energy utilized to produce acoustic signals. Second, excessive heat may damage the components or electrical interconnects of the loudspeaker and/or degrade the adhesives often employed to attach various components together, and may even cause the loudspeaker to cease functioning. Increase of the voice coil temperature is accompanied by the increase of the voice coil's direct current resistance (DCR). Since all modern amplifiers are sources of voltage, the increase of DCR causes the decrease of sound pressure level (SPL) output. A voice coil temperature of 250 C corresponds to approximately double the DCR and, correspondingly, −6 dB drop in SPL which is also accompanied by a change of frequency caused by undamping of the loudspeaker's motor.
As additional examples, the voice coil may become detached from the coil former and consequently fall out of proper position relative to other components of the transducer, which adversely affects the proper electromagnetic coupling between the voice coil and the motor assembly and the mechanical coupling between the voice coil and the diaphragm. Also, excessive heat will cause certain magnets to become demagnetized. Thus, the generation of heat limits the power handling capacity and distortion-free sound volume of loudspeakers as well as their efficiency. Such problems are exacerbated by the fact that electrical resistance through a voice coil increases with increasing temperature. That is, the hotter the wire of the voice coil becomes, the higher its electrical resistance becomes and the more heat it generates.
The most common form of loudspeaker uses a single voice coil winding in a single magnetic air gap. However, loudspeaker performance may be enhanced by using a multiple coil/multiple gap design. A multi-coil transducer may include two or more separate windings axially spaced apart from each other to form two or more coils which are usually electrically connected so that the coils work together to move the diaphragm. As both coils provide forces for driving the diaphragm, the power output of the loudspeaker may be increased without significantly increasing size and mass. Many multi-coil/multi-gap designs are able to produce more power output per transducer mass and dissipate more heat than conventional single-coil designs. For example, a dual-coil design provides more coil surface area compared with many single-coil configurations, and thus is capable of dissipating a greater amount of heat at a greater rate of heat transfer.
While the multiple coil/multiple gap construction has several advantages over single coil/single gap designs including higher power handling, reduced distortion, reduced inductance, and extended frequency response, there are several disadvantages with dual coil/dual gap speakers. First, insofar as a desired advantage of the dual-coil transducer is its ability to operate at a greater power output, operating the dual-coil transducer at the higher power output concomitantly causes the dual-coil transducer to generate more heat. As such, the improved heat dissipation inherent in the dual-coil design may be offset by the greater generation of heat. There can also be problems with overheated magnets due to the compact magnet assembly and the proximity of the magnets to the heat-generating voice coils. For example, as compared to single-coil transducers, adequate heat dissipation in many dual-coil transducers is a problem due to the longer thermal paths that must be traversed between the voice coil and the ambient environment.